This invention relates to the production of plywood sheets used for cabinet making and the like and more particularly it relates to the production of blanks, i.e., a composite of several wood veneer sheets to which finish or facing sheets are applied.
Plywood sheets as contemplated for this invention consist of multiple veneer sheets, a typical number being seven sheets, that are glued together to produce a plywood sheet, e.g., a xc2xe inch thick sheet that is 4 feet wide and 8 feet long. The outer or facing sheets of veneer are high quality hardwood that provide a decorative appearance. The inner sheets can be a variety of wood species and provide structural integrity. Hereafter it is to be understood that reference to plywood production applies to such plywood sheets having decorative facing and is used to produce cabinetry and the like as opposed to plywood used for construction, i.e., a base sheet for roofing (to which shingles are applied) or flooring (to which carpeting may be applied) and the like.
A plywood manufacturer has in the past produced plywood in a continuous assembly line operation wherein multiple veneer sheets are essentially glued, stacked, pressed, trimmed and then sanded. More recently there have been attempts to separate the process into two stages. One stage is the lamination of the inner ply sheets to form what is referred to as a blank, with the second stage merely applying the facing sheets to the opposed sides of the blank. This separation of the process provides both advantages and disadvantages. The facing sheets which are substantially more expensive than what is required for the inner sheets (the blank) can be much thinner than the inner sheets as long as sanding is held to a minimum, e.g., limited to polishing. Further, the blanks can be produced as a common plywood core to which different decorative facing sheets are applied, e.g., oak, alder, birch, maple, etc. To benefit from the advantages as explained for separating the process, the blank needs to be produced to very exacting tolerances and at a low cost. Prior to this invention, the process of blank production has required substantial manual operations that are cost prohibitive and it is an objective of the present invention to produce the blanks at a reduced cost through automation of the process.
A criteria for the blank production is that the outer plies are substantially free of irregularities such as cracks or niches or whatever that may affect the final appearance after a paper thin sheeting of facing veneer is applied. Accordingly, in the initial assembly process, the outer plies of the blank are selected by grade and are generally of higher quality with respect to such defects than the inner plies of veneer.
As a second criteria for this assembly process, because the veneer sheets are produced from wood which has grain direction, and in accordance with industry practice, the plies or sheets are laid up to have alternating grain direction. Still further to enable dimensioning of the blank in the finish operation where the blank is sanded (abrasive planing) the assembled sheets for the blank are required to have an overall thickness that exceeds the desired end thickness. It is, however, further desired that the added thickness not result in unnecessary waste. It has been determined that the desired added thickness to enable efficient dimensioning is about 0.040 inch. The veneer sheets are provided with varying thicknesses (e.g., {fraction (1/10)} inch, ⅙ inch and {fraction (1/7)} inch) to enable assembly to the desired thickness.
It will accordingly be appreciated that the sequencing of the sheets is a complex operation. A plurality of stacks of veneer sheets are provided, with the sheets of each stack being similar in grade, grain direction and thickness and the different stacks achieving a desired mix of these properties. For a particular blank, the two outside sheets are to be the better quality. The combined thicknesses of the sheets in whatever order is to exceed the desired end thickness by 0.040 inch. The assembly of sheets is to have alternating direction of grain.
As for example for the production of a five sheet blank, the assembly procedure requires the stacking of four sheets (the bottom sheet being a high quality sheet) with each sheet having its upper side coated with glue and then adding an unglued top sheet to the stack (also a high quality sheet). For this preferred embodiment, four sheets of the five sheet blank are selected and placed on a conveyor in the desired sequence (the first sheet being an outside sheet, the second, third and fourth sheets having alternating grain direction, the fourth sheet having a grain direction opposite to the top sheet, and the four sheets when combined with the top sheet having the desired thickness). Providing this desired sequence (for the preferred embodiment) is achieved using a plurality of feeder machines (e.g., four machines sometimes referred to as robots) each of which is directed by computer control to a stack of the sheets and, in the desired sequence, lays the sheets on a conveyor.
It is important that these feeder machines operate without error. Previous machines used for a somewhat similar purpose included overhead conveyors where the top sheet of a stack was pulled upwardly to the conveyor by suction and conveyed to the conventional bottom supporting conveyor belt. However, such transfer mechanisms were not able to achieve the desired error free transfer. The feeder machines otherwise available included a carrier member having a plurality of suction cups. The suction cups are lowered onto a stack of veneer sheets and grip the sheets and roboticly move sheets one by one to the assembly line conveyor. Such machines were prone to misfeeding and were too slow. The misfeeding of the sheets can be explained in part by an appreciation that the sheets that are laid in the stack present an undulating surface to the suction members (cups) and because the sheets have defects, (cracks, etc.), the suction members that are positioned over a defect or undulation often fail to produce the required air tight grip. These problems were addressed for the present invention by using smaller and greater number of suction cups. The heads include (in the preferred embodiment) as many as 50 or more cups arranged in five rows of 10 cups each. Each cup can adjust to different surface heights and each cup is designed to conform to undulations. Only about half the number of working cups applied are needed to consistently grip and transfer the sheets and whatever the defect in a given sheet (a sheet considered acceptable for such plywood production), there are sufficient suction cups not affected to achieve gripping.
The desired speed of the transfer machines is achieved by use of multiple carriers or gripping heads (e.g., four) and then the use of linear actuator for moving the carriers from the stack to the conveyor. The four gripping carriers are individually and cooperatively lowered onto the selected stacks to grip and remove the top sheet off each stack which are then rapidly moved to the conveyor by the linear actuator to be released onto the conveyor in rapid error free succession. The above operation of the robots is controlled by a computer which directs each carrier to the desired stack in the desired sequence, the sequencing of the sheets referred to as the recipe of the blank.
Following laying of the sheets onto the conveyor, the sheets are conveyed through a glue application station, assembled into five sheet stacks in accordance with the sequence as presented by the conveyor, (such assembly referred to as a rough blank) and thereafter cured, trimmed and sanded, e.g., in accordance with industry practice. The invention here is directed to the assembly process whereby the sheets are arranged in a desired sequence and deposited onto a conveyor for presentation to a lay up station where the sheets, following glue application, are assembled into a rough blank.
The production of the rough blank will be more fully understood upon reference to the following detailed description having reference to the accompanying drawings.